1869.] DAWKINS BRITISH POSTGLACIAL MAMMALS. 213 



Could the Megarhine or Southern species of Rhinoceros have dwelt 

 under the same climate as the Musk -sheep ? I think this altogether 

 impossible. Had the constitution of the former animal been suffi- 

 ciently elastic, it would have held its own ground against the in- 

 vading animals from the north and east after the manner of its 

 feUow the R. leptorhinus of the French and Italian Pliocenes. 

 There is, however, one view that has the merit of explaining the 

 difficulty, and which therefore is probably true. During the de- 

 pression of North Germany and the greater portion of Britain, those 

 parts of the Preglacial continent now represented by France and 

 the south of England were not submerged ; for in that case they 

 would present some traces of the deposit of icebergs, which were so 

 numerous in the North Sea of the period. It is hardly within 

 reason to suppose that all proof of submergence beneath the Glacial 

 sea could have been removed by subaerial denudation from so large 

 an area, while to the north of the Thames, and in North Germany, 

 it is so abundant and so ample. It is therefore probable that the 

 valley of the Lower Thames roughly marks the position of the 

 ancient Glacial coast-line in Britain, and that to the south the relics 

 of the Preglacial continent extended without a break through France 

 into Italy ; while to the north the look-out was over a dreary expanse 

 of sea burdened with icebergs, like that off the coast of Newfound- 

 land. 



The temperate or moderately warm climate that prevailed over 

 the British portion of the Preglacial continent before the depression 

 took place, must have been lowered by the presence of so much 

 melting ice as is implied by the presence of the Boulder-clay, and 

 especially in the neighbourhood of the coast-line, independently of 

 any great change flowing from some other unknown and cosmical 

 cause. This climatal change must have banished to a certain extent 

 the Preglacial mammals from the area over which it was felt ; but 

 nevertheless it is highly consistent with what we know of the 

 migration of Herbivores to suppose that now and theu some of them, 

 such as R. megarhinus, may have ventured northwards as far as the 

 valley of the Thames. 



Again, M. Lartet has shown, in his memorable essay on the mi- 

 gration of mammals, that the Arctic division of the Postglacial or 

 Quaternary mammalia (the Mammoth and Tichorhine Rhinoceros 

 and the rest) invaded Europe from their ancient home in the north 

 of Asia (where they probably dwelt during the Pliocene epoch, 

 Tertiary, in "Western and Central Europe) at the commencement of 

 the European Quaternary or Postglacial period, the change in the 

 Pliocene temperature, coupled very possibly with the depression of 

 land in North Siberia, causing the animals inhabiting that area to 

 advance westwards and southwards and to occupy the feeding- 

 grounds till then belonging to the Pliocene fauna. This immigra- 

 tion very probably began while Northern Europe was heing depressed 

 beneath the waves in the Boulder-clay epoch. If this be admitted, 

 there is nothing improbable in the hypothesis that the North Asiatic 

 immigrants would gradually creep round the shores of the Glacial 



q2 



